INTEENAL PEOTECTIVE SECEETIONS 



in structure (as well as function, infra) from the accessory thyroids. 

 They were first described by Sandstrom (1880) in man and certain 

 other mammals, externally 

 to the lateral lobes of the 

 thyroid body. Subsequent 

 observations confirmed their 

 constant presence in mam- 

 malia, adding to the external 

 parathyroids other similar 

 little glands situated on the 

 mesial surface of the lateral 

 lobes of the thyroid, the 

 internal parathyroids, which, 

 however, may be absent in 

 certain species (Fig. 3). 



In man the outer (also 

 called the inferior) para- 

 thyroids lie in front of the 

 inferior thyroid artery and 

 the recurrent nerve. Their 

 position is not constant. 

 For the most part they are 

 situated at the inferior \"^. 

 angle of the thyroid lobes, 

 towards the lower part of 

 the postero-external border, 

 at a greater or less distance 

 from it, and closely united 

 by fine connective tissue. 

 More rarely they are found at the level of the eighth and tenth 

 tracheal ring (Fig. 4). It follows that in excising the thyroid body 

 in man by the subcapsular method, the inferior or outer para- 

 thyroids are easily left in situ a fact which, as we shall see, is 

 of great clinical and physiological importance. 



The inner (or superior) parathyroids are situated on the internal 

 surface, towards the upper pole of the thyroid lobes, with which 

 they are intimately connected, since they are wrapt in a common 

 sheath of connective capsular tissue, and sometimes lie in the 

 depth of the thyroid substance. In surgical thyroidectomy these 

 must obviously be excised along with the thyroid body. 



The structure of the parathyroids (both outer and inner) 

 differs from that of the principal and accessory thyroids. They 

 consist not of hollow vesicles, but of compact masses or columns 

 of epithelium cells, which sometimes anastomose into branching 

 cords. Between the cell masses there are septa of connective 

 tissue, which convey the blood-vessels and nerves into the gland 

 substance (Fig. 5). 



Fio. 3. Transverse section of left lobe of thyroid from 

 a two-months' kitten. (Kohn.) a, thyroid tissue ; 

 b, thymic tissue ; p, p', inner and outer para- 

 thyroids. 



